A storage system is a computer that provides storage service relating to the organization of information on writable persistent storage devices, such as memories, tapes or disks. The storage system is commonly deployed within a network attached storage (NAS) or storage area network (SAN) environment. A SAN is a high-speed network that enables establishment of direct connections between a storage system, such as an application server, and its storage devices. The SAN may thus be viewed as an extension to a storage bus and, as such, a storage operating system of the storage system enables access to stored information using block-based access protocols over the “extended bus”. In this context, the extended bus is typically embodied as Fibre Channel (FC) or Ethernet media (i.e., network) adapted to operate with block access protocols, such as Small Computer Systems Interface (SCSI) protocol encapsulation over FC or Transmission Control Protocol/Internet Protocol (TCP/IP)/Ethernet.
SCSI is a peripheral input/output (I/O) interface with a standard, device independent protocol that allows different peripheral storage devices, such as disks, to attach to the storage system. In SCSI terminology, clients operating in a SAN environment are initiators that initiate requests and commands for data. The storage system is a target configured to respond to the requests issued by the initiators in accordance with a request/response protocol. The SAN clients typically identify and address the stored information in the form of blocks or disks by logical unit numbers (“luns”).
When used within a NAS environment, the storage system may be embodied as a file server including a storage operating system that implements a file system to logically organize the information as a hierarchical structure of directories and files on, e.g., the disks. Each “on-disk” file may be implemented as a set of data structures, e.g., disk blocks, configured to store information, such as the actual data for the file. A directory, on the other hand, may be implemented as a specially formatted file in which information about other files and directories are stored.
The file server, or filer, of a NAS system may be further configured to operate according to a client/server model of information delivery to thereby allow many client systems (clients) to access shared resources, such as files, stored on the filer. In the client/server model, the client may comprise an application executing on a computer that “connects” to the filer over a computer network, such as a point-to-point link, shared local area network, wide area network or virtual private network implemented over a public network, such as the Internet. The clients typically communicate with the filer by exchanging discrete frames or packets of data according to pre-defined protocols, such as the TCP/IP.
NAS systems generally utilize file-based protocols to access data stored on the filer. Each NAS client may therefore request the services of the filer by issuing file system protocol messages (in the form of packets) to the file system over the network. By supporting a plurality of file system protocols, such as the conventional Common Internet File System (CIFS), the Network File System (NFS) and the Direct Access File System (DAFS) protocols, the utility of the filer may be enhanced for networking clients.
A common type of file system is a “write in-place” file system, an example of which is the conventional Berkeley fast file system. In a write in-place file system, the locations of the data structures, such as inodes and data blocks, on disk are typically fixed. An inode is a data structure used to store information, such as metadata, about a file, whereas the data blocks are structures used to store the actual data for the file. The information contained in an inode may include, e.g., ownership of the file, access permission for the file, size of the file, file type and references to locations on disk of the data blocks for the file. The references to the locations of the file data are provided by pointers, which may further reference indirect blocks that, in turn, reference the data blocks, depending upon the quantity of data in the file. Changes to the inodes and data blocks are made “in-place” in accordance with the write in-place file system. If an update to a file extends the quantity of data for the file, an additional data block is allocated and the appropriate inode is updated to reference that data block.
Another type of file system is a write-anywhere file system that does not over-write data on disks. If a data block on disk is retrieved (read) from disk into memory and “dirtied” with new data, the data block is stored (written) to a new location on disk to thereby optimize write performance. An example of a write-anywhere file system that is configured to operate on a filer is the Write Anywhere File Layout (WAFL™) file system available from Network Appliance, Inc. of Sunnyvale, Calif. The WAFL file system is implemented within a microkernel as part of the overall protocol stack of the filer and associated disk storage. This microkernel is supplied as part of Network Appliance's Data ONTAP™ storage operating system residing on the filer.
Data is often received at the storage system from the network as packets of various lengths that are stored in lists of variable length input buffers. In contrast, file systems usually operate on data arranged in blocks of a predetermined size. For instance, data in the WAFL file system is stored in contiguous 4 kilobyte (kB) blocks. Therefore, data received by the storage system is typically converted from variable length input buffers to the fixed sized blocks for use by the file system. The process of converting data stored in input buffers to fixed sized blocks involves copying the contents of the input buffers into the system's memory, then having the file system reorganize the data into blocks of a predetermined size. However, the copy operation from the input buffers to the file system buffers consumes processor resources as that copy operation is performed in software. The present invention is directed to a technique that eliminates this copy operation into the file system buffers.